The match ended in a dull 1-1 draw as Tammy Abraham's late goal for the visitors was cancelled out by Sassuolo's Andrea Pinamonti.
"And it disappoints me because the effort the squad made was betrayed by one player because of an unprofessional attitude. He betrayed the effort everyone else made," Mourinho said.
The Portuguese manager declined to name the player when asked who it was that betrayed the team.
"No. I have already told him in the changing room – something I don’t often do because normally after games I don’t speak in the changing room. I had 16 players out there tonight and I was happy with the attitude of 15 of them," he added.
"But an unprofessional attitude, an incorrect attitude compared to his teammates – that’s what annoys me. But, it’s a point, a point away from home, and a result you can’t say is negative. I was generally pleased with the attitude of the side."
Roma play Torino at home on Sunday in their last Serie A game before the World Cup break.